1. Field of the Invention
The present invention relates generally to a light wave distance measuring apparatus and, more particularly, to a light wave distance measuring apparatus for measuring a distance by comparing a phase difference between a measuring signal relative to distance measuring light and a reference signal.
2. Related Background Art
FIG. 3 is a block diagram showing one example of construction of a conventional light wave distance measuring apparatus.
An oscillator 101 in the form of a crystal oscillator outputs a clock CLK having a frequency as high as 45.0 [MHz]. A frequency dividing circuit 102 (103, 104) divides the frequency of the clock CLK, thereby creating a modulation signal FT having a relatively high frequency of 15.0 [MHz] and a low reference signal IF having a relatively low frequency of 3.66 [KHz].
A Phase-Locked Loop (PLL) circuit 107 outputs a high reference signal FR having a frequency deviating by the frequency of the low reference signal IF from the frequency of the modulation signal FT, i.e., a relatively high frequency of 14.99633 [MHz] by employing the low reference signal IF as an input signal.
A light emitting element 106 emits the distance measuring light which is intensity-modulated by the modulation signal FT. A light receiving element 112 receives a part of this distance measuring light that is reflected by a known reflector member (unillustrated). The light receiving element 112 outputs a light receiving signal SFT into which the received distance measuring light is photoelectrically converted. A mixer 113 mixes the frequencies of the light receiving signal SFT and the high reference signal FR, thereby creating an intermediate signal SIF. A frequency of this intermediate signal SIF is substantially equal to a frequency of a difference between the frequency of the light receiving signal SFT and the frequency of the high reference signal FR, i.e., the frequency of the low reference signal IF. A bandpass filter (BPF) 114 improves an SN ratio of this intermediate signal SIF. A wave shaping circuit 115 transforms the intermediate signal (analog waveform) passing through the BPF 114 into a predetermined pulse waveform.
A phase measuring circuit 116 compares a phase of the intermediate signal SIF with a phase of the low reference signal IF. The phase measuring circuit 116 outputs a phase difference signal indicating a phase difference between these two compared signals. A microprocessor 117 converts this phase difference signal into a distance to the reflection member in accordance with the frequency of the low reference signal IF employed. The distance is indicated on an indicator 118.
In the case of this light wave distance measuring apparatus, the frequencies of the respective signals are set to satisfy the following conditions:
fFT=b.times.fIF PA1 fFR=(b+1).times.fIF, or (b-1).times.fIF
where fFT is the frequency of the modulation signal FT, fIF is the frequency of the low reference signal IF, fFR is the frequency of the high reference signal FR, and 1/b (b is the integer) is the frequency dividing ratio of the frequency divider.
By the way, this type of the light wave distance measuring apparatus generally includes two internal and external optical paths between the light emitting element and the light receiving element. The internal optical path is conceived as a reference optical path, having a fixed optical path length, along which a beam of light emitted from the light emitting element falls directly on the light receiving element or via the internal optical system. On the other hand, the external optical path is defined as a distance measuring optical path along which the beam of light emitted from the light emitting element outgoes from the apparatus body, subsequently is reflected by the reflection member and incident on the light receiving element.
The light wave distance measuring apparatus described above has such a defect that an error in measurement is induced by fluctuations in phase of the signal due to a built-in electric circuit. Accordingly, for eliminating this error, the prior art light wave distance measuring apparatus is constructed as below.
To be specific, a first optical signal passing through the external optical path and a second optical signal passing through the internal optical path are selectively incident on the light receiving element. First and second light receiving signals photoelectrically converted based on the incident light are thereby obtained. The first and second light receiving signals are converted into first and second intermediate signals having low frequencies. Phases of the first and second intermediate signals are compared with a phase of the reference signal having the low frequency, thus obtaining distances of the internal and external optical paths, respectively. Herein, the distance obtained by use of the second light receiving signal is likely to be a distance of the internal optical path. The distance of the internal optical path is a known value from the design. Hence, if an error is caused between the known distance of the internal optical path and the measured distance of the internal optical path, this error can be conceived as an error due to the electric circuit. Paying attention to this, the error of the electric circuit is obtained, the distance of the external optical path that is obtained from the first light receiving element is compensated by the error of the electric circuit, thereby obtaining a distance to the reflection member.
However, the conventional light wave distance measuring apparatus still, though constructed in the manner described above, contains an error in the distance measured. This error includes an error caused by the PLL circuit.
Next, a PLL circuit 117 will be described in greater detail. A phase comparator 108 makes a comparison between phases of two signals, i.e., a low reference signal IF and another input signal, viz., a feedback signal IF'. The phase comparator 108 outputs a voltage signal (DC voltage) indicating a phase difference between the two signals. A low-pass filter (LPF) 109 smooths the voltage signal and outputs the smoothed voltage signal. A voltage control type oscillation circuit (VCO) 110 outputs a high reference signal FR having a predetermined frequency fFR (high frequency) in proportion to the voltage signal. The frequency of the high reference signal FR is 14.996 [MHz] obtained by subtracting the frequency of the low reference frequency IF from the frequency of the modulation signal FT. A frequency divider 111 divides the frequency of the high reference signal FR. The frequency divider 111 creates a feedback signal IF' having a frequency fIF' substantially equal to the frequency fIF of the input signal IF and outputs this feedback signal to the phase comparator 108.
As explained earlier, the prior art light wave distance measuring apparatus is so set that the signals (actually, the signals having the same frequency) having the low frequency are inputted to the phase measuring circuit and the PLL circuit as well. It is because the input signals having the low frequency are required for facilitating the creation of the high reference signal FR having the frequency fFR deviating by the frequency fIF of the base intermediate signal from the frequency fFT of the modulation signal in the PLL circuit.
However, the PLL circuit 107 is, in principle, easy to undergo a frequency modulation (FM) depending on the frequency of the input signal (low reference signal IF) of the phase comparator 108. Because of this fact, the output signal (high reference signal FR) of the PLL circuit 107 turns out a signal containing the signal FR having the oscillation frequency fFR of the VCO 110 and side band spurious signals FR-IF and FR+IF which deviate by the frequency fIF of the input signal IF from the frequency fFR of the signal FR. Namely, the output signals of the PLL circuit become, as illustrated in FIG. 4B, three signals (1) FR, (2) FR-IF and (3) FR+IF each having a different frequency. These spurious signals cause the error in the measurement. More specifically, the side band spurious signal consisting of (2) FR-IF and (3) FR+IF is mutually modulated with the high reference signal FR when transmitted via a non-linear element such as a mixer 113. Further, the side band spurious signal is frequency-mixed with the light receiving signal SFT. As a result, the side band spurious signal turns out a signal (hereinafter called a low spurious signal) SIF' having a frequency fSIF' (lower frequency) containing no phase information. That is, the mixer 113 mixes the frequencies of the light receiving signal SFT into which the distance measuring light is photoelectrically converted and of the 3-wave component output signals FR, FR+IF and FR-IF containing the spurious signals SIF'. Consequently, as illustrated in FIG. 4C, the low spurious signal SIF' containing no distance information is superposed on the intermediate signal SIF.
According to the conventional light wave distance measuring apparatus, the phase measuring circuit 116 effects a phasic comparison between the frequency of the intermediate signal SIF containing this low spurious signal SIF' and the low frequency fIF of the reference signal IF. This results in such a problem that the phase difference obtained contains an error, and the measured value of distance contains an error.
Then, a variety of contrivances have been performed to lessen those errors in the conventional light wave distance measuring apparatus. For instance, a time constant of the LPF 109 of the PLL circuit is set large to eliminate the spurious components contained in the input signal of the VCO 110. Alternatively, a variable range of the frequency of the output signal of the VCO 110 is narrowed. Variations in the output signal of the VCO 110 are thereby reduced even when the input signal of the VCO 110 changes due to the spurious components.
If the time constant is set large, however, there arises a problem in which stabilization of the PLL circuit is time-consuming when turning ON the power supply and switching over the operation of the PLL circuit. In addition, narrowing the frequency variable range of the output signal of the VCO 110 requires the use of an oscillator exhibiting a high stability wherein the oscillation frequency is hard to change even if variations in the voltage of the power supply are caused due to temperature. The problem is that such an oscillator is too expensive.
Further, for decreasing the mutual modulation in the mixer 113, input levels of the light receiving signal SFT as an input signal to this circuit and of the reference signal FR have to be optimized by a cut-and-try method such as a test. An adjustment thereof is troublesome.